The present invention relates generally to power distribution systems and, more particularly, to a system and method for detecting high resistance ground faults in a power distribution system and protecting the power distribution system from such ground faults upon detection thereof.
A typical power distribution system includes a converter, an inverter and a mechanical load such as a motor. The converter is typically linked to a three phase source that provides three phase AC power and converts the three phase power to DC power across positive and negative DC buses. The DC buses feed the inverter which generates three phase AC power on output lines that are provided to the load. The inverter controls the three phase AC voltages and currents to the load so that the load can be driven in a desired fashion. Cables connect the power source to the converter and also connect the inverter to the load.
One common type of three-phase power distribution system is an adjustable speed drive (ASD). ASDs are frequently used in industrial applications to condition power and otherwise control electric driven motors such as those found with pumps, fans, compressors, cranes, paper mills, steel mills, rolling mills, elevators, machine tools, and the like. ASDs typically provide a volts-per-hertz or vector controls and are adept at providing variable speed and/or variable torque control to an electric driven motor, such that ASDs have greatly improved the efficiency and productivity of electric driven motors and applications.
Power distribution systems such as ASDs require protection from inadvertent cable and load (e.g., motor) failures which can lead to undesirable ground faults. The root cause of cable failures is often cable insulation breakdown and therefore most ground faults occur in the cables between the power source and the converter or between the inverter and the load. When a ground fault occurs, the results can be extremely costly. For instance, ground faults often result in power interruptions, equipment failure and damage, uncoordinated system decisions with potential for overall plant interruptions, degraded or lost production and overall customer frustration.
Resistance grounding systems are used in industrial electrical power distribution facilities to limit phase-to-ground fault currents. Generally speaking, there are two types of resistors used to tie a power distribution system's neutral to ground: low resistance and high resistance. High Resistance Ground (HRG) systems limit the fault current when one phase of the system shorts or arcs to ground, but at lower levels than low resistance systems. In the event that a ground fault condition exists, the HRG typically limits the current to 5-10A, though most resistor manufacturers label any resistor that limits the current to 25A or less as high resistance.
HRG systems are commonly seen in industrial applications where continued operation is important to the process, such as in power distribution systems where any power source downtime has a dramatic economic cost. HRG systems have gained popularity in such applications due to their ability to continue operation in lieu of a single line-ground fault and improved ability to limit escalation of the single line-ground fault into a multi-phase event. Additionally, HRG systems function to suppress transient line to ground over voltages during a ground fault, eliminate arc flash hazards with phase to ground faults, and reduce equipment damage at the point of ground fault.
Since ground fault conditions in HRG systems do not draw enough current to reliably trigger fault current sensors in an associated motor drive, ground fault detections systems must be employed to detect HRG faults. Various such ground fault detection systems and methods have previously been implemented to locate ground faults. For example, in US Publication No. 2009/0296289, detection of a ground fault in the HRG system is accomplished by injecting a common mode voltage into the three phase system and measuring the system response, with the sensed output voltages then being filtered to determine the HRG fault occurrence. In another example, and as set forth in US Publication No. 2009/0080127, detection of a ground fault in the HRG system is accomplished by measuring the DC bus voltage in the HRG system. However, while such systems function to detect a ground fault in the HRG system, the methods employed in those systems are either computationally cumbersome or intrusive to the system. Additionally, existing ground fault detection systems and methods fail to locate the ground fault in the HRG system (i.e., identify where and which phase the fault occurs on). As such, challenges remain in HRG systems with respect to identifying HRG faults in a cost effective manner and locating the HRG fault in the system.
It would therefore be desirable to provide a system and method that provides a computationally efficient approach to detect an HRG fault in a three-phase power distribution system and identify the HRG fault location in a particular phase.